1. Field of the Invention
This invention generally relates to thin-film integrated circuits and, more particularly, to micro-electro mechanical system (MEMS) structures capable of converting thermal energy to electrical energy.
2. Description of the Related Art
Every year, billions of dollars' worth of energy is thrown away in the form of waste heat. For example, 25% of the energy generated from a car's combustion engine produces motion, and through the alternator, generates electricity to power electronic accessories. The remaining 75% of the energy is lost through heat. Significant efforts have been made in the study of heat harvesting, motivated by both the desire to address alleged climate change issues and the challenge of energy autonomy in wearable wireless transducer nodes.
Thermoelectric (TE) generators are the state of the art in heat harvesting technology, to directly convert heat to electricity. However, the technology still suffers from low conversion efficiency and high costs, due to the fact that rare earth materials (e.g., bismuth telluride) are required. Further, these materials have low thermal insulating properties. Other products are being developed. BMW recently announced the use of 24 TE modules on a car exhaust that generated 600 watts of electrical power under motorway driving conditions, which was 30% of car's electrical requirement. Currently, the typical conversion efficiency of a TE device is in the range of 10 to 20 microwatts per square centimeter (uW/cm2). Any improvement on the conversion efficiency would result in significant module size reduction and cost savings.
Besides the waste heat from industries, cars, and household appliance, the human body readily generates heat on the average of 5.3 mW/cm2. Converting human body heat to electricity is a very attractive concept, since it would enable self-powered wireless health monitoring technology, which could makes the system not only handy and autonomous, but also especially useful in emergencies and when help is difficult to reach. By using a thermoelectric generator attached to human body, some simple self-powered wearable wireless sensing systems have already been attempted. Apparently, the small temperature difference between the human body and ambient temperature, and the limited conversion efficiency of the TE devices, has as of yet made wearable wireless health monitoring systems impractical.
FIGS. 21A and 21B are, respectively, a cross-sectional view of a basic MEMS bimorph cantilever and a strain energy chart (prior art). The cantilever is composed of 3 materials: a high value coefficient of thermal expansion (CTE) upper film, a low value CTE lower film, and a low thermal conductivity material as anchor. In some aspects, the low value CTE film and anchor may be the same material. When heated, the cantilever bends downward due to the CTE mismatch at the interface of the upper film and lower film, and when cooling the cantilever moves back to its original position.
If arranged between a hot surface and a cold surface as illustrated, the MEMS bimorph cantilever vibrates as follows:                1. the cantilever is exposed to a heated surface and initially its tip touches the hot surface;        2. the body of cantilever is heated, the cantilever bends downward and, as a result, its tip separates from the hot surface and the heating stops;        3. as the cantilever cools down the downward bend is relaxed and the cantilever moves back to its original position;        4. the system returns to Step 1, and the next cycle begins.        
Although a vibration can be established thermally, the very small deflection displacement implies that the induced strain energy is too low to be meaningful for energy harvesting, as depicted in the strain energy chart (FIG. 21B).
It would be advantageous if small thermal-to-electrical energy conversion devices could be made more efficient.